Acoustic transducers of a conventional electro-acoustic system, such as loudspeakers, play a fundamental and significant role in an audience's listening experience. The loudspeakers are, in fact, the critical link between the electrical signal representing audio information and the resulting audio signal heard by the listener. Consequently, the performance of an electro-acoustic system may be severely limited by its loudspeakers. The loudspeakers of an electro-acoustic system must reproduce sound throughout the audio spectrum, which is typically considered to be from 20 Hz to 20,000 Hz. It is difficult for a single loudspeaker to accurately reproduce sound over the entire frequency range. Several loudspeakers are typically used in such a situation to provide adequate volume and coverage of the audio spectrum. Each loudspeaker is dedicated to reproducing sound for a particular frequency range, and the complement of loudspeakers are coordinated through a crossover network. However, electro-acoustic systems having multiple loudspeakers pose a particular challenge to the audio engineer.
In the case where the multiple loudspeakers are physically disassociated and may be placed at different positions throughout the listening area, the issue of time coherency between the audio signal generated by each loudspeaker becomes particularly significant. That is, the position of each loudspeaker relative to one another is related to the difference in time for the resulting audio signal produced by the respective loudspeaker to reach a particular point in the listening area. It is quite often the case where the poor quality of sound is not due to the quality of the loudspeaker itself, but is a result of the multiple loudspeakers not being aligned in time effectively for the majority of the audience. Thus, one of the goals of an acoustic engineer is to arrange and correct for the various positions of the loudspeakers of the electro-acoustic system in order to produce a coherent wavefront. The benefits of time correcting multiple loudspeakers located in acoustic space include minimized comb filtering, reduction of reverberant field, and increased intelligibility of the acoustic signal.
Practically speaking, it is often not possible to physically position multiple loudspeakers in relation to one another to produce a coherent wavefront. The positioning of loudspeakers may be limited by the physical space available for placement of the loudspeakers, as well as the size and shape of the listening area in which the loudspeakers are placed. To accommodate the various placement of loudspeakers in relation to one another, programmable electronic delay circuits have been used to correct for time disparities between the loudspeakers. The delay circuits may be programmed to delay the arrival of the stimulus signal to one loudspeaker, with respect to another loudspeaker, so that the difference in their relative position may be compensated by the programmed time delay. Thus, a more coherent wavefront of the resulting audio signal may be produced. However, in order for this method of time correction to produce sufficient results, it is necessary to determine the relative position of the multiple loudspeakers. The relative position of the multiple loudspeakers may be determined by the relative time delay of the acoustic signals of each loudspeaker.
One method of determining the relative time delay of the multiple loudspeakers is to physically measure the distance from each of the loudspeakers to a point located in the listening area. The relative time delay of each loudspeaker may be calculated from the resulting measurement, and used to program the appropriate delay times of the delay circuits. However, this method does not acknowledge the fact that the distance of each loudspeaker should be measured from its respective “acoustic center.” The acoustic center of a loudspeaker is a term used to note the actual sonic origin of sound from the loudspeaker. The acoustic center is typically located further away than the loudspeaker itself. Thus, measuring the physical distance of the actual loudspeaker will not necessarily coincide with the physical distance of its acoustic center. In programming the delay times to produce a coherent wavefront, the distance should be measured from the acoustic center.
Further complicating the measurement is the fact that the acoustic center of a loudspeaker is frequency dependent. Due to the electro-mechanical nature of conventional loudspeakers, its acoustic center shifts depending on the frequency of the audio signal being produced. Consequently, the relative distances of multiple loudspeakers will change throughout the audio spectrum. Another factor that should be considered, but cannot be determined from physical measurement, is additional delay introduced by the electro-acoustic system itself, for example, digital signal processing of the stimulus signal prior to providing the resulting analog signal to the loudspeakers.
Another factor affecting the quality of sound the audience experiences is the polarity of a particular loudspeaker with respect to the stimulus signal, as well as to the other loudspeakers. The polarity of a loudspeaker is determined by its connection to the power amplifier of the electro-acoustic system. Two loudspeakers connected to have opposite polarities will produce audio signals 180 degrees out of phase. Consequently, the resulting audio signals may destructively interfere with one another and affect the overall sound quality. Determining the polarity of a particular loudspeaker by visually inspecting its connection to the power amplifier may not be practical if the loudspeaker is located in a position that is difficult to reach. For example, the speaker may be located high above the listening area, or may be mounted into a wall. In either case, visually inspecting the connection of the loudspeaker will not be easy.
There currently exists analysis equipment for evaluating various performance characteristics of an electro-acoustic system and its loudspeakers. One such system is described in U.S. Pat. No. 5,555,311 to Reams, issued Sep. 10, 1996. The system described in the Reams patent can determine, among other things, the bandwidth, the thermal limit, and the group delay of an electro-acoustic system. A Tef System and SIM System II are some additional examples of analysis equipment. These tools perform Fast Fourier Transforms on an impulse stimulus yielding a complete time and frequency analysis. Another measurement tool for loudspeaker evaluation and room acoustic which uses maximum length sequences is known as MLSSA. However, determining the time delay of the electro-acoustic system and its loudspeakers using these equipment involve interpreting data provided in a format more suited for discerning other measurement data. For example, measurement data may be provided in the form of a graphical representation of an impulse response. Determining the time delay involves interpreting the graphical information, which may require special training to understand the resulting measurement data. Furthermore, the existing analysis equipment often involve complicated and time-consuming setup procedures, which may also be carried out only by specially trained technicians.